Various methods of creating pores with a small cross-section in sheets of polymer material are already known in the prior art, for example with a view to producing microporous membranes for the purification or filtering of industrial or biological fluids, or for water treatment. These methods can be grouped together according to three major types:                a first, mechanical, type of method comprising at least one stamping step, as described for example in the document U.S. Pat. No. 4,652,412;        a second type of method, comprising at least one irradiation using a CO2 or NdYAG infrared laser or pulsed laser, as described for example in the documents U.S. Pat. No. 4,923,608, U.S. Pat. No. 3,742,182, WO-A-98 30317;        a third type of method, comprising at least one ion irradiation followed by a chemical etching.        
The method according to the invention for creating pores in a material such as polycarbonate in sheet form belongs to the third general type presented above. For this type of pore creation method, with a view to producing filtration membranes, reference can be made for example to the following documents: DE-A-4 319 610, U.S. Pat. No. 5,234,538, U.S. Pat. No. 3,713,921. The document U.S. Pat. No. 4,956,219, from the applicant, describes a method of creating pores in a material chosen from amongst the group comprising saturated polyesters such as ethylene polyterephthalate, carbonic acid polyesters such as polycarbonate produced from bis-phenol A (bis(hydroxy-4 phenol)-2,2 propane), aromatic polyethers, polysulphones, polyolefins, cellulose acetates and cellulose nitrates. The material is bombarded by a beam of ions preferably issuing from rare gases such as argon, with an energy of around 2 MeV per nucleon, the density of ions passing through the polymer film being between 104 and 1013 ions per square centimeter. Such beams can be obtained by means of particle accelerators such as cyclotrons with separate sectors. The material is in the form of a strip moving substantially perpendicular to the beam of ions, the thickness of the strip being from around a few microns to 100 microns, the width of the strip being between 5 and 150 centimeters. By magnetic deflection, the beam of ions effects a sinusoidal sweep, each portion of the strip being bombarded on several occasions so that an even density of pores is obtained over the entire strip of material treated. After bombardment, the strip of material is possibly subjected to ultraviolet (UV) radiation. After this UV treatment or directly after ion bombardment, a chemical treatment is effected in a corrosive solution containing an organic solvent. Thus, for example, the strip of material is immersed in a solution of caustic soda containing methanol, ethanol or isopropanol. The ion bombardment and/or the chemical treatment can be carried out continuously, possibly one directly after the other, the strip of material which passed opposite the beam being driven continuously in the corrosive solution. After neutralisation, rinsing and drying, a continuous strip of microporous polymer material is obtained.
The document U.S. Pat. No. 3,852,134 describes a method for the ion bombardment of polycarbonate film with a thickness of less than 20 microns, followed by exposure to radiation with a wavelength of less than 4000 angstroms, under oxygen, before a first chemical etching, after baking and second chemical etching with a view to obtaining pores with diameters of between 1000 and 100,000 angstroms. The preferential etching methods in directions defined by molecular structure defects resulting from an ion bombardment make it possible to produce filtering membranes with a greater quality than the membranes resulting from other methods such as stamping or laser treatment. However, controlling the density, shape and size of the pores obtained is still tricky. Thus for example there is a probability that one or more pores may pass completely through the membrane which, in some applications, may be detrimental. To reduce this risk, a method of bombardment on both faces of the membrane is proposed in the document U.S. Pat. No. 4,855,049. This method does however result in an unfavourable hydromechanical behaviour in some cases, because of the great convolution obtained for the fluid passages. It has also been found that the pores are of variable diameter from the surface towards the heart of the membrane, thus having a “cigar” shape (for polycarbonate membranes, see Schönenberger et al., J. Phys. Chem. B101, p. 5497-5505, 1997). This in particular interferes with a good prediction of the properties of these membranes merely looking at their surface, for example with a scanning electron microscope. The cause of this shape of the pores is still being discussed.
The document U.S. Pat. No. 3,713,921 presents the use of a surfactant added to the etching reagent in order to attenuate these variations in shape and transverse dimension of the pores. Some authors invoke an influence of the thickness of the membrane and imperfect control of the etching conditions in order to explain the “cigar” shape of the pores.
The invention relates to a method of creating pores in a polymer material in sheet form, such as polycarbonate or any other equivalent material, the said method making it possible to obtain porous areas with controllable sizes and shapes, these areas being distributed according to densities and locations which can also be controlled. According to one embodiment, the method also allows, within the said areas, the formation of pores of a cylindrical shape overall, without any depreciable variation in average diameter of these pores in the thickness of the sheets of polymer material treated. The invention also concerns the microporous membranes produced from the said treated sheets of polymer material.
The invention relates, according to a first aspect, to a method for creating pores with a nanometric to micrometric size in a polymer material in a thin sheet which can be supported, comprising an ion bombardment followed by chemical etching, the said method comprising a step of global heat treatment providing partial deactivation of the traces formed in the polymer material by the ion bombardment, and a step of selective irradiation of the polymer film, steps which take place after the ion bombardment and before the chemical etching.
In another embodiment, the global heat treatment and the selective irradiation of the bombarded polymer material are carried out simultaneously. In one embodiment, the selective irradiation is effected after the heat treatment of the bombarded polymer material. In another embodiment, the selective irradiation is effected by means of a UV source and through a mask. In another embodiment, the selective irradiation is effected by means of a UV laser beam. According to one particular embodiment, a step of pre-etching of the polymer material is carried out prior to the ion bombardment, this pre-etching reducing the thickness of the sheet of polymer material.
The polymer material is chosen from the group comprising saturated polyesters such as ethylene polyterephthalate, carbonic acid polyesters such as polycarbonate produced from bis-phenol A (bis(hydroxy-4 phenol)-2,2 propane), aromatic polyethers, polysulphones, polyolefins, cellulose acetates and cellulose nitrates. The sheet of polymer material initially has, and in particular before any pre-etching, a thickness of between a few microns and around a hundred microns. The pre-etching is carried out until the ablation of a thickness of between 0.5 microns and 3 microns approximately on each face of the said sheet. According to a particular embodiment, the polymer material is an amorphous polycarbonate approximately 25 microns thick before pre-etching. According to another particular embodiment, the polymer material is a crystalline polycarbonate with a thickness of approximately 10 microns. The ion bombardment is performed by a beam of ions preferably issuing from rare gases such as argon, with an energy of around 2 MeV per nucleon, the beam having an intensity of between 106 and 1013 ions per second.
In one embodiment, the chemical etching is said to be slow and is carried out in a bath containing 0.5 N caustic soda in aqueous solution, at a temperature of approximately 70° C., for approximately 260 min. In another embodiment, the chemical etching is said to be fast and is carried out in a bath containing 2 N caustic soda, in aqueous solution, at a temperature of approximately 70° C., for approximately 30 min. The chemical etching bath comprises, in one embodiment, an organic solvent chosen from amongst the group comprising methanol, ethanol and isopropanol. The chemical etching is carried out in the presence of a surfactant. The microporous films obtained after chemical etching are washed until the pH is neutralised, rinsed and dried. The washing of the microporous film is carried out in an aqueous solution of acetic acid at approximately 15%, at a temperature of approximately 70° C. for approximately 15 minutes; then in demineralised water, at a temperature of approximately 70° C., for approximately 15 minutes and more, until a neutral pH is obtained.
The method for creating pores described above is carried out continuously. The invention relates, according to a second aspect, to a microporous film of polymer material produced by implementing the method presented above. The microporous film is used as a matrix with a view to producing micrometric filaments of metal or polymer.